This invention relates to a burst control circuit for use in a time division multiple access (TDMA) communications system.
One of the prior arts of this invention is disclosed in U.S. Pat. No. 5,101,176 entitled "BURST CONTROL CIRCUIT FOR USE IN TDMA COMMUNICATIONS SYSTEM" which is assigned to the same assignee as the present application, and which will hereafter be called the prior circuit.
In the prior circuit, a power amplifier having two control terminals amplifies an input radio signal to produce an output burst signal. A level control signal representing a difference between a reference signal and the level of the output burst signal, feedback-controls the level of the output burst signal through a first control terminal of the amplifier. A waveform control signal of a fixed amplitude controls the waveform of the output burst signal through a second control terminal of the amplifier.
The waveform control signal has a trapezoidal waveform and the rise and fall times of the waveform control signal are so determined that the spectrum bandwidth of the output burst signal is suppressed within an allowable range.
A non-linear power amplifier having a high power efficiency is used as the power amplifier of the prior circuit. In a small power output range of this non-linear power amplifier, a small increment of the level control signal causes an excessively large increment of the output burst signal.
This characteristic of the power amplifier makes it difficult to carry out a fine level control in a small power output range. And this excessively high gain of the amplifier in a small power range often deteriorates the stability of the feedback-control, resulting in overshoots in the leading and trailing edges of the waveform.